1. Field of the Invention
This invention pertains in general to a synchronization method and system for digital receivers and, more particularly, to a method and system for obtaining frame, carrier, and clock synchronization for digital audio broadcasting and digital video broadcasting systems.
2. Description of the Related Art
Digital audio broadcasting and digital video broadcasting provide high quality signals with minimal multi-path distortion, noise, or signal drop-offs, as commonly seen in analog broadcasting. There have been various attempts to standardize a digital audio broadcasting ("DAB") system for mobile, portable, and fixed receivers. One such standard is the Eureka 147 system. Eureka is a research and development consortium of European governments, corporations, and universities to develop new technologies. Project number 147, began in 1986, focuses on the DAB technology and seeks to implement the technology under a standardized system for both terrestrial and satellite broadcasting. The Eureka 147 system has been adopted in Europe and Canada, and is being considered in the U.S. in conjunction with the "In band, On channel" ("IBOC") system.
An audio input signal of the Eureka 147 DAB system is generally encoded with a masking-pattern universal subband integrated coding and multiplexing ("MUSICAM") to divide a signal into subbands and to minimize hearing threshold and masking to achieve data reduction. Convolutional coding is then performed, followed by re-distribution of data bits in time and frequency, i.e., interleaving, such that the largest error that can occur in a signal block is limited and the ability to correct burst errors is increased. In addition, the Eureka 147 system uses orthogonal frequency division multiplexing ("OFDM") with quadrature phase shift keying ("QPSK") modulation on each carrier to distribute several audio data channels among overlapping carriers. A guard interval is also inserted between data blocks to reduce intersymbol and multipath interference. The modulated DAB signals are then transmitted over multipath channels.
As with many communication systems, synchronization for a DAB system at various communication layers, especially frame, carrier, and clock synchronization at the physical layer, is important. Because of various changes in propagation channels for a DAB system, synchronization is difficult, especially for carrier and clock synchronization. In addition, many algorithms and systems for conventional OFDM systems are incompatible with the Eureka 147 system largely due to the difference in the transmission frame structure of the Eureka system.
As an example, Fouche et al. describe a method and apparatus for realigning local oscillators to lower the complexity of the clock recovery system at the receiver level in U.S. Pat. No. 5,313,169, entitled "Method of Realigning the Local Oscillators of a Receiver and Device For Implementing the Method", issued on May 17, 1994. The method and device are based on two master lines of the OFDM signal spectrum having a fixed frequency difference between them. This difference is used to calculate the variation of the phases of the lines. However, the frequency spectrum of the Eureka 147 system does not have the master lines as described in Fouche et al.
Leuing et al. describe a method and apparatus for frame synchronization in a pure ALOHA system in U.S. Pat. No. 5,444,697, entitled "Method and Apparatus for Frame Synchronization in Mobile OFDM Data Communication", issued Aug. 22, 1995. The method and apparatus require a three-stage synchronization process. An incoming OFDM signal is first detected. The second step in the process is to sample the received signal and measure the correlation, preferably carried out in the frequency domain, between the signal and a reference signal to achieve coarse synchronization. Finally, the synchronization process is completed by calculating the time-shift between coarse and actual synchronization points to determine phase correction to apply to each sub-carrier. However, the described method and apparatus are different from the Eureka 147 system in that in the ALOHA system, synchronization of each OFDM frame is required, and therefore each frame must carry its own synchronization data. In addition, the described method and apparatus do not provide for carrier or clock synchronization.
Philips describes a "System for Broadcasting and Receiving Digital Data, Receiver and Transmitter for Use in Such System" in U.S. Pat. No. 5,550,812 issued on Aug. 27, 1996. Each frame of the digital data in the system described in Philips includes multicarrier data and system symbols, and each symbol includes a set of OFDM carriers at carrier positions within a frequency raster with regular carrier spacing. The frames further include AFC symbols, or frequency reference symbols, each having at each reference peak position a unmodulated carrier having a peak signal power level. The data frame structure described is also different from that of the Eureka 147 system.
In addition, conventional methods and systems for achieving carrier frequency synchronization first estimate the integral frequency offset of the carrier frequency and then compensate for the fractional frequency offset. A method of achieving synchronization of an OFDM is described by Keller et al. in "Orthogonal Frequency Division Multiplex Synchronization Techniques for Wireless Local Area Networks", PIMRC '96, October 1996. However, Keller et al. do not disclose a method and system for synchronizing the Eureka 147 system because Keller et al., use a special signal format absent from the Eureka 147 system.